Strange power supply problem

[ruffrecords]

I am busy commissioning the EZTubeMixer demo mixer and I am having a strange power supply related problem. With just a couple or three channel modules plugged in, everything works fine. As soon as I plug a fourth in I get a large output buzz at about -30dBu. Looking on the scope it is exactly 100Hz but half the waveform is dead flat and the other half is a high frequency - can't trigger well enough on that portion of the waveform to see exactly what frequency. I disconnected the HT supply and you can plug in all 6 modules and it is fine which at least demonstrates it is probably not the regulated 12V supply hooting. I took out all the first stage tubes (12AX7) and you can plug in all six modules and you do not get the oscillation so it is either getting into the first stage or the first stage is creating it. With four modules fitted and only one fitted with its 12AX7 the oscillation occurs. I added a 10K across the HT to pull another 30+mA from the HT supply and repeated the above test. No oscillation. Added a 12AX7 to a second module  and you get the oscillation.

I am going to add some small caps across the NFB resistor to see if this is self oscillation but I would welcome any other ideas.

Cheers

Ian

 

[mjrippe]

Ian, although I don't feel qualified to give *you* advice, here are a few ideas off the top of my head:
Have you measured the current draw on the HT line?  Is it higher than expected?
Perhaps leave the first stage tubes in place and remove second stage tubes and see what happens?  (Or is the 2nd stage the output stage?)
Is there any way to tell if the oscillation is in the modules or the PSU? 
Grounding?

 

[bruno2000]

Regulator starvation?
Best,
Bruno2000

 

[ruffrecords]

Thanks guys for the ideas. I have made a little progress. Grounding is always a potential cause. The power supply for this mixer is situated at the front and the dc supplies travel by cables to the motherboards at the rear. I needed to make a connection from the HT 0V at the power supply to the mains safety earth tag which is next to the mains inlet connector at the rear. My normal rule is to take a wire direct from the HT- to the  safety earth tag but, as the end of the HT run through the motherboards is right close to this tag I decided to connect from the motherboard HT 0V to the safety earth tag. I did noy tink it would make any difference. However, when checking the heater elevation voltage at the heater supply I connected a DVM from the heater -ve to a different earth connection that is used for the screen between windings in the HT transformer. With just two 12AX7s fitted, connecting the DVM to make this measurement stopped the interference dead in its tracks. Aha, I thought, grounding problem. So I disconnected the lead from the motherboard HT 0V to safety earth and made a connection direct to the the PSU PCB HT 0V. Switched on and it was dead quiet. So I added a couple more 12AX7s and tried again with 4 modules. I was very disappointed to find the interference had returned.

So, just to make sure it was not a problem with my Lindos test set, I unplugged it and tried again. With all four channels fully populated with tubes there was no buzz. Plugging in the signal source from the Lindos increased it a little but plugging in the output brought it back fully. I disconnected the mixer output from the the Lindos and plugged in a passive VU meter - and the buzz returned. With only the input or the output connected there is full buzz but with just one there is only a small buzz. This happens to all four channel amp line inputs and direct outputs.

So, I feel as though I am getting closer but I don't yet have the answer. Next test is to make sure I have not make some silly error in wiring up the XLRs.

Cheers

Ian

 

[ruffrecords]

I contacted Holger about this problem and it turned out he had experienced something similar. His cure was to add decoupling caps to the HT supply on each of the motherboards. I did this and I am pleased to say it works perfectly. Many thanks Holger.

Full details on the blog here:

http://eztubemixer.blogspot.co.uk/

Cheers

Ian

 

[mjrippe]

Excellent!  Glad you got it sorted and thanks for sharing all of your work.